Metal coating



Jan. 18, 1949. w HARRls HAL 2,459,161

METAL COATING Fil ed Jan. 1:5, 1945 T I QNEY.

Patented Jan. 18, 1949 ma'mncoa'rmo Arch W. Harris, Cleveland, Ohio, and Alfred B.

. Ward, Pittsburgh. Pa., assignors to The Ameri-,

can Steel and Wire Company of New Jersey, a corporation of New Jersey and Carnegie-Illinois Steel Corporation, a corporation of New Jersey Application January 13,1945, Serial No. 572,728

3 Claims. (Cl. 117-51) This invention is particularly concerned with continuously coating steel strip with molten metal of the class consisting of zinc, tin, aluminum and their alloys, but certain of its features may be applicable to coating in a manner other than continuously.

The accompanying drawing schematically illustrates a specific example of the invention in operation.

More specifically, this drawing shows steel strip l, which may be of any chemical composition, gauge and width suitable for coating with molten metal of the class described,this strip being upwardly fed continuously from a coil 2 around a roller 3 and downwardly into a tank 4 containing a water solution of ammonium chloride, otherwise called sal ammoniac, the strip being dipped into the solution in this tank by a sinking roller 5 in such a fashion that both sides of the strip become thoroughly wet with the solution. The ammonium chloride concentration of this solution may vary from 1 to by weight, the balance being water.

Leaving the ammonium chloride solution in the tank 4, the strip next travels between squeegee rollers 6 pressed against both sides ofthe strip and having surfaces proper for squeegeeing such as soft rubber, the squeegeeing action being ad- Justed to leave the strip wet on both sides with thin smooth films of sal ammoniac solution. The water from these films is then evaporated, by the strip next passing over a heater 1, so as to leave its surfaces coated with dry ammonium chloride. The heater 1 may be of the open gas-flame type since the temperatures to which it heats the strip are not critical beyond the limits required to thoroughly dry its surfaces, by evaporation of.

the water, but, since the purpose in mind is to provide the strip with uniformly distributed layers of ammonium chloride, it is important that the temperature of the strip is not raised so high as to cause vaporization of the ammonium chloride.

With its dried coatings of ammonium chloride, the strip next enters and passes through a horizontal enclosure or mufile 8 from which it leaves over a roller 9, this roller 8 and the squeegee rollers 6 guiding the strip so it travels through the mufile as a free span, the strip then bending downwardly around this roller 8 and going around and under a sinking roller l0 so as to dip into the described molten coating metal in a pan II, the strip traveling upwardly from the sinking roller l0 over a roller l2 and downwardly to a coiler ii, the latter pulling the strip under tension from the coil 2.

The end of the muiiie 8 opens into a box ll completely covering the top of the coating metal, into which the strip dips, the roller 8 and the strip itself, the idea being to completely enclose the strip from the time it enters the muflle 8 until it leaves the coating metal. To this end, the

edges of the box It dip into the top of the metal in the pan H, the box being made gas-tight and connecting in a gas-tight manner with the muffle 8.

A considerable portion of the muiile 8 is surrounded by a heating chamber I5 designed to put sufiicient heat into the mufile to raise the temperature of the strip traveling through it to from 1000 F. to higher temperatures, the maximum temperature being fixed by the fact that it must not be soextreme as to melt the strip. The portion of the muilie thus heated is that portion toward the end of the mufile where the strip enters it, and a suificient length of the muflle at 1 its other end is left exposed to the air to provide for cooling the strip to temperatures required for the hot-dip coating application, it being understood that the diflerent' metals in the class described require diflerent temperatures depending on the peculiarities of the particular metal involved. Generally speaking, the prior art is familiar with the temperatures required, in the case of any of the metals described, for the proper alloy-bonding responsible for the adherence of the coating to the strip. In all cases the length of the muflie enclosed by the furnace so it functions as a heating zone, and the length left exposed to the atmosphere so that it functions as a cooling zone, may be determined by any skilled furnace designer, due consideration being paid respecting the gauge of the strip, its traveling speed, and

the temperature to which it is desired to be raised in the heating portion of the mume.

A gas inlet i6 is provided in the box H at its end remote from the muiile 8 and, preferably, quite close to the top of the coating metal enclosed by this box, hydrogen being introduced through this inlet under sufficient pressure to cause the hydrogen to flood the box it and flow through the muilie 8 counter the traveling direction of the strip, the strip-entering-end of the muilie being preferably provided with a seal I1 and a little stack l8 close to the seal H, the stack l8 providing a gas exit for the muiiie and also serving to provide an indication that the hydrogen is flowing through the furnace, since the exiting gases may be lit at this stack to indicate the presence of the hydrogen by continued buming of a flame. In such cases the flame should burn continuously since this assures the passage of hydrogen through the muille, the hydrogen dried ammonium chloride enters the muflle and soon acquires a high enough temperature to curse vaporization of the ammonium chloride, thus continuously introducing ammonium chloride to the muflie at its strip-enteringend. The chlorine in the ammonium chloride vapor, thus formed in the muflle, functions to condition the strip for the coating step, it being the chlorine that apparently is effective in carrying out this conditioning and it being possible to introduce chlorine in other forms. to the muflle, such as by the injection of hydrogen chloride or, for that matter, chlorine gas somewhere near its strip-enteringend. However, the introduction of the ammo-' nium chloride in the manner described is .very advantageous in that it is continuous and permits easy and exact control of the amount of ammonium chloride introduced. It may be nientioned at this'point that if too thin ammonium chloride layers are formed on the strip prior to its entering the muflie, as by the use of too low an ammonium chloride concentration in the water solution or by too tightly adjusting the squeegee rollers, that then there will not be enough ammonium chloride to provide for effective conditioningfor application of the coating, experience best indicating the minimum amount needed but it being suggested that in actual practice the amount should not drop below around .002 pound per hundred square feet of strip surface area. On the other hand, too heavy a coating of ammonium chloride produces a roughly coated product, experience again best serving to indicate when too much ammonium chloride is used. Generally speaking, the amount should not exceed .015 pound per hundred square feet of the strip surface.

Now if the ammonium chloride vapor concentration in the muffle is suflicient to properly condition the strip for its ultimate coating with the molten metal, of the class described, the iron of the strip is attacked by the chlorine. That is to say, to perform its function the chlorine concentration, whether provided as ammonium chloride, hydrogen chloride, or chlorine, must be suflicient to form compounds with the iron of the strip.

This-is disadvantageous as an ultimate result since it represents an iron loss and causes complications during the application of the molten coating metal. Furthermore, in commercial work it is ordinarily desirable to make the muflle out of a ferrous material, and this muflle issubject to chemical attack when the chlorine concentration of the muffle atmosphere, through which the strip is being passed, is sumcient for its intended purpose.

zinc chloride is one of the fluxes used in ordinary zinc coating practices, but this requires a thick layer of the flux and it has been found that when the fluxis not used, and the chlorine contacts 5 the zinc; that the thin layer of zinc chloride that then forms introducw roughness to the ultimately coated product.

All of the foregoing disadvantages, connected with the use of chlorine in the muflie, are eliminated in the case of the present invention by the introduction of hydrogen through the inlet I 6 to the box H, the flow rate and pressure of the hydrogen being adjusted to keep a barrier of the hydrogen between the surface of the metal in the pan II and the ammonium chloride, or other chlorine-containing atmosphere, in the muflle, this barrier consisting of hydrogen alone in the box I I and at least an adjacent part of the cooling portion of the muflle 8, and then gradually becoming mixed with the ammonium chloride in the heated portion of the muiile.

Now in all instances the hydrogen must be suflicient to render the atmosphere throughout the heated portion of the muflle' reducing respecting the formation of the compounds that would otherwise form between the chlorine and the iron of the strip under treatment. When vthis is done, iron-chlorine compounds either do not form or are immediately reduced, and by keeping the atmosphere reducing, throughout the length of the mufile, respecting the formation of compounds that would otherwise be formed in the absence of the hydrogen between the chlorine and form, conditions the strip for reception of the molten coating metal in the pan l l as effectively under reducing conditions as under any other condition. -It also follows that after the strip is conditioned, and during its cooling to a temperature permitting application of the molten coating metal, itis continuously bathed in a reducing atmosphere.

It is to be understood that it is unnecessary to use pure hydrogen, it being possible to introduce a mixture of hydrogen and a comparatively inert gas, such as nitrogen, to the inlet l6. However, in all instances there must be sufficient hydrogen to maintain the atmosphere throughout the muflle reducing in nature respecting the formation of the iron-chlorine compounds that would form otherwise. v

After the strip has passed through the reducing atmosphere of hydrogen and ammonium chloride, or hydrogen and chlorine, and is cooled, by passing through. the cooling portion of the mufiie, it then enters the molten coating metal and emerges therefrom with a properly bonded coating, the roller I! being arranged sufliciently high to permit complete freezing of the metal. It may be desirable to wipe the strip, as it leaves the coating bath, to more uniformly distribute the coating.

Although 1000 F. has been set as the minimum temperature to which the strip must be heated,

this applies mainly to the continuous process specifically disclosed. But it may be that the principles of the invention might otherwise be used advantageously as by discontinuous methods providing longer treatment times than are possible with practical continuous methods. In such instances the temperature of the work while it is being treated in the atmosphere containing the chlorine and hydrogen may be dropped as low as 660 F., this being at least academically operative assuming that adequate time is permitted. When the strip is on the move, as it is in a continuous process, it has not been found posisble to drop below around about 1000 F., and l050 F. may be taken as a completely safe working minimum temperature in any event.

It is to be noted that in all instances it is possible to use a treating temperature below the recrystallization temperature of cold-worked steel strip and, likewise, well below the critical temperature of ordinary steel strip. Therefore, deadsoft box-annealed steel strip may be coated with metal in the manner described without its being subjected to the equivalent of a normalizing heat treatment, whereby it largely retains its deadsoft characteristics. On the other hand, fullhard cold-rolled strip may be coated without substantial loss of its full-hard characteristics. At the same time the temperatures may be raised well above the 1000 F. minimum, applicable to the continuous process, so as to eflect normalizing or annealing of full-hard cold-rolled strip as a part of the metal coating process, whereby to combine the heat treatment and metal coating of cold-reduced steel strip into one process.

The product resulting from the present invention has exceptionally good adherence between its metal coating and the steel strip forming the base, the adherence being equivalent to that obtained by the old method of hot-dipping sheets singly. Furthermore, the steel may vary in its composition to a greater extent than has ever before been possible, commercially satisfactory adherence between zinc and the steel strip being attained even when the strip is made from rimmed steel with its characteristically nearly pure iron surfaces that have always caused poor zinc adherence heretofore.

We claim:

1. A ferrous metal coating method comprising wetting the ferrous metal with a water solution of ammonium chloride, drying the metal so it retains a coating of ammonium chloride, heating the metal to temperatures ranging from 660 F. to higher temperatures insufficient to melt the ferrous metal in an atmosphere containing sufficient hydrogen tomake it reducing respecting the formation of iron compounds by the ammonium chloride, and subsequently coating the ferrous metal with molten metal selected from the group consisting of zinc, tin, aluminum and their alloys.

2. A continuous ferrous metal strip and wire product coating method comprising continuously passing the product through a water solution of ammonium chloride, drying the product so it retains a coating of ammonium chloride, continuously passing the thus coated product through an enclosure containing hydrogen at temperatures of from 1000 F. to higher temperatures insufllcient to melt the product, and subsequently continuously coating the product with molten metal selected from the group consisting of zinc, tin, aluminum and their alloys.

3. A continuous steel strip and wire product coating method comprising continuously passing the product through a bath of a water solution of ammonium chloride to wet the product therewith, drying the product so it retains a coating of ammonium chloride, continuously passing the thus coated product through an enclosure while heating the product in the product entering end of the enclosure to temperatures of from 1000 F. to higher temperatures insufflcient to melt the product so as to vaporize its said coating and produce an atmosphere of ammonium chloride vapor in the enclosure, coating the product at the product exiting end of theenclosure with molten metal selected from the group consisting of zinc, tin, aluminum and their alloys with the molten coating metal exposed to the enclosure atmosphere, and continuously introducing hydrogen to the product exiting end of the enclosure to protect the coating metal from the ammonium chloride and so the hydrogen flows to the product entering end of the enclosure and mixes with the atmosphere there to maintain it reducing respecting the formation of compounds of iron and chlorine at the temperatures described.

' ARCH W. HARRIS.

ALFRED H. WARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

